1. Field of the Invention
The invention relates to pressure swing adsorption systems. More particularly, it relates to improved adsorbent beds for use in such systems for air separation operations.
2. Description of the Prior Art
Cryogenic air separation plants utilize ultra-cold temperatures to separate oxygen and nitrogen from air. Such plants typically utilize low amounts of power, but have high capital costs associated therewith. Because of such high capital costs, cryogenic air separation plants are competitive only for high flow capacity operations.
For lower capacity operations, where cryogenic air separation plants may not be economically feasible, pressure swing adsorption (PSA) systems are particularly suitable for a wide variety of important commercial applications. For example, high purity oxygen is used in various industries, such as chemical processing, steel mills, paper mills, and in lead and gas production operations. Nitrogen is also used in numerous chemical processing, refinery, metal production and other industrial applications.
In the PSA process, a feed gas mixture, such as air, containing a more readily adsorbable component and a less readily adsorbable component, e.g., the nitrogen and oxygen components of air, is passed to the feed end of an adsorbent bed capable of selectively adsorbing the more readily adsorbable component at a higher adsorption pressure. The less readily adsorbable component passes through the bed and is recovered from the discharge end of the bed. Thereafter, the bed is depressurized to a lower desorption pressure for desorption of the more readily adsorbable component, and its removal from the feed end of the bed prior to the introduction of additional quantities of the feed gas mixture for repressurization and adsorption as cyclic adsorption-desorption-repressurization operations are continued in the bed. Such PSA processing is commonly carried out in multi-bed systems, with each bed employing the PSA processing sequence on a cyclic basis interrelated to the carrying out of such processing sequence in the other beds of the adsorption system. In PSA systems for the recovery of high purity oxygen product as the less readily adsorbable component of air, each adsorbent bed will commonly contain an adsorbent material capable of selectively adsorbing nitrogen as the more readily adsorbable component, with the selectively adsorbed nitrogen being subsequently desorbed and recovered from the bed upon reduction of the pressure of the bed from the higher adsorption pressure level to a lower desorption pressure level. PSA systems for the recovery of nitrogen product have likewise been based on the use of adsorbents that selectively adsorb nitrogen from air as the more readily adsorbable component thereof.
Early PSA air separation systems utilized two and three beds, with well known molecular sieves, e.g., 13X zeolite molecular sieve material, being used as the adsorbent therein. Such zeolitic molecular sieve material, and other such materials, e.g., 5A material, capable of selectively adsorbing nitrogen from air, are equilibrium type adsorbents. Thus, an adsorption front of the selectively adsorbed nitrogen is formed at the feed end of the bed of such material, and advances toward the discharge or oxygen product end as a result of the equilibrium conditions established in the bed of zeolite molecular sieve material between the more readily adsorbable nitrogen and the less readily adsorbable oxygen components of feed air.
While conventional zeolite molecular sieves can desirably be used in PSA operations, specially modified materials can also be used for the desired selective adsorption of nitrogen from feed air, and the recovery of oxygen or nitrogen as the desired product gas. Thus, the lithium cation forms of conventional zeolite X have, more recently, been developed for use in PSA processing. Such lithium, i.e. LiX, adsorbent is found to exhibit a highly desirable capacity and selectivity toward the adsorption of nitrogen from air or other streams containing less polar or less polarizable molecular species, such as oxygen.
LiX adsorbent materials proposed for PSA processing operations are the lithium cation forms of zeolite in which the framework Si/Al.sub.2 molar ratio is from about 2.0 to about 3.0, preferably from 2.0 to 2.5, and in which at least about 88%, preferably at least 90%, more preferably at least 95% of the AlO.sub.2 -tetrahedral units are associated with lithium cations. The nitrogen adsorption properties of such highly exchanged forms of LiX are totally unpredictable from the results obtainable using LiX materials in which 86 equivalent percent or less of the cations are lithium and the remainder are principally sodium cations.
It had been found that, when said 13X adsorbent material was combined with two-bed vacuum PSA (VPSA) systems, desirably lower power requirements and capital costs could be achieved. Such vacuum systems are typically operated with a higher adsorption pressure above atmospheric pressure, and a lower desorption pressure in the subatmospheric pressure range. The newer, specially modified adsorbent materials referred to above have been found to greatly lower the power requirements of the VPSA system. However, because of the more complex processing requirements needed to produce such specially modified adsorbent materials, the costs associated with the use of such materials, hereinafter referred to as "special adsorbent" materials, are quite high, and VPSA processing employing such special adsorbents tends to be very expensive. In typical size VPSA systems, two beds of approximately six foot lengths are filled with conventional 13X adsorbent or with special adsorbent material. While such VPSA systems, using the special adsorbent have on the order of 15% lower power costs than those using conventional 13X adsorbent, such special adsorption systems have a considerably higher, adsorbent material cost than the conventional VPSA system of the same size.
While the special adsorbent materials have desirable properties for use in VPSA systems, it will be seen that the high cost of such specially modified materials creates a deterrent to the use thereof in practical commercial VPSA systems. It would be highly desirable in the art for developments to be made leading to improved VPSA processing, particularly developments enabling such special adsorbent materials to be more fully utilized in VPSA systems without the appreciable cost disadvantage presently associated therewith.
It is an object of the invention, therefore, to provide improved VPSA systems and processes.
It is another object of the invention to provide adsorbent beds for VPSA systems capable of achieving a desirable balance of lower power requirements and reduced costs.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.